This invention relates generally to astable multivibrators and particularly to astable multivibrators that are utilized in applications involving operation at two displaced frequencies.
The invention is specifically adapted for use in connection with deflection circuits for video monitors having the capability of operating at two deflection frequencies, depending upon the degree of resolution desired in the display. Video monitors incorporate a cathode ray tube (CRT) that has a deflection yoke that is supplied with deflection voltages developed by a horizontal output transistor and a deflection transformer. The horizontal output transistor is driven from an oscillator source that is synchronized or keyed to incoming video information by an appropriate synchronizing signal, generally referred to as horizontal sync pulses. In many monitors, a display of higher resolution is made available. Conventional monitor deflection frequencies are similar to those used in the display of television signals, namely, about 15.7 KHz. Higher frequency monitor displays may be on the order of 18 KHz. Still other monitors utilize display frequencies of about 22 KHz.
A video display monitor designated ZMM1470G, manufactured by ZENITH DATA SYSTEMS, a subsidiary of ZENITH ELECTRONICS CORPORATION, the assignee of the present application, incorporates an astable multivibrator circuit. The monitor has two display frequencies, one approximating 16 KHz and the other approximating 22 KHz, which are selectable by operation of a suitable switch.
As is well known, astable multivibrators generally perform best when the transistor that supplies the base drive to the horizontal output transistor is conducting about 60% of the time, that is, has a 60% duty cycle. Difficulty may be experienced when switching between two widely displaced deflection frequencies in a monitor having an astable multivibrator. Often the system cannot react quickly enough to enable the astable multivibrator to attain proper sync. Indeed, under certain conditions, the circuit can actually fail to achieve synchronization and go into a lock-up mode.
To achieve the desired duty cycle for the horizontal output transistor in a prior art monitor, a duty cycle compensating circuit was used which, in response to a voltage change in the high voltage regulator, changed the resistance in the RC network supplying one of the transistors in the astable multivibrator. The regulator sensed a voltage change when switching between the deflection frequencies and reacted to maintain an essentially constant high voltage. The sensed change was used to operate a transistor switch circuit for changing the resistance in the RC time constant circuit and alter the duty cycle of the astable multivibrator to maintain the desired duty cycle of the horizontal output transistor more closely. That circuit worked well for deflection frequency changes between 15 and 18 KHz. That circuit, however, would not correct for the problem encountered when switching between deflection frequencies of 15 KHz and 22 KHz. In those instances, the astable multivibrator would often experience a condition where it did not lock in sync, since the sync signals did not occur during the "on" time of the proper one of its transistors.